Detector

ABSTRACT

A detector for detecting the presence of a memory tag, the detector comprising a radio frequency source operable to generate a radio frequency signal and a detector resonant circuit part connected to the radio frequency source, the detector resonant circuit part comprising an antenna,
         the detector further comprising a power monitor responsive to the power of a reflected signal returned from the detector resonant circuit part,   the power monitor being operable to generate an output in response to the power of the reflected signal,   wherein a decrease in the power of the reflected signal indicates the presence of a tag in the vicinity of the antenna.

This application is a Continuation of U.S. Ser. No. 10/697,269 filed onOct. 31, 2003, which claims priority from Great Britain patentapplication 0227196.3, filed on Nov. 21, 2002. The entire content of theaforementioned applications are incorporated herein by reference

FIELD OF THE INVENTION

This invention relates to a detector for detecting the presence of amemory tag, a read/write device, a read/write system and a method ofdetecting the presence of a memory tag.

BACKGROUND OF THE INVENTION

Memory tags in the form of Radio Frequency Identification (RFID) tagsare well known in the prior art, and the technology is well established(see for example: RFID Handbook, Klaus Finkenzeller, 1999, John Wiley &Sons). RFID tags come in many forms but all comprise an integratedcircuit with information stored on it and a coil which enables it to beinterrogated by a read/write device generally referred to as a reader.Until recently RFID tags have been quite large, due to the frequencythey operate at (13.56 MHz) and the size of coil they thus require, andhave had very small storage capacities. Such RFID tags have tended to beused in quite simple applications, such as for file tracking withinoffices or in place of or in addition to bar codes for productidentification and supply chain management.

Much smaller RFID tags have also been developed, operating at variousfrequencies. For example Hitachi-Maxell have developed “coil-on-chip”technology in which the coil required for the inductive link is on thechip rather than attached to it. This results in a memory tag in theform of a chip of 2.5 mm square, which operates at 13.56 MHz. Inaddition Hitachi has developed a memory tag referred to as a “mu-chip”which is a chip of 0.4 mm square and operates at 2.45 GHz. These smallermemory tags can be used in a variety of different applications. Some areeven available for the tagging of pets by implantation.

Although it is known to provide tags with their own power source, inmany applications the tag is also powered by the radio frequency signalgenerated by the reader. Such a known system is shown in FIG. 1 where areader is indicated generally at 10 and a tag at 12. The reader 10comprises a radio frequency signal generator 13 and a resonant circuitpart 11, in the present example comprising an inductor 14 and acapacitor 15 connected in parallel. The inductor 14 comprises a antenna.The resonant circuit part will have a particular resonant frequency inaccordance with the capacitance and inductance of the capacitor 15 andthe inductor 14, and the frequency signal generator 13 is operated togenerate a signal at that resonant frequency.

The tag 12 similarly comprises a resonant circuit part generallyillustrated at 16, a rectifying circuit part generally indicated at 17and a memory 18. The resonant circuit part 16 comprises an inductor 19which again comprises in this example a loop antenna, and a capacitor20. The resonant circuit part 16 will thus have a resonant frequency setby the inductor 19 and capacitor 20. The resonant frequency of theresonant circuit part 16 is selected to be the same as that of thereader 10. The rectifying part comprises a forward-biased diode 21 and acapacitor 22 and thus effectively acts as a half-ware rectifier.

When the reader 10 is brought sufficiently close to the tag 12, a signalgenerated by the frequency generator 13 will cause the resonant circuitpart 11 to generate a high frequency electromagnetic field. When theresonant circuit part 16 is moved within this field, a current will becaused to flow in the resonant circuit part 16, drawing power from thetime varying magnetic field generated by the reader. The rectifyingcircuit part 17 will then serve to smooth the voltage across theresonant frequency part and provide a power supply storage. Therectifying circuit part 17 is sufficient to supply a sufficiently stablevoltage to the memory 18 for the memory to operate.

There are many uses to which such memory tags may be put. For example inEP 1 076 316 A2 an application is described whereby a memory tag isattached to a print of a photograph and contains data about the printconcerned.

For some uses the RFID tags are attached to the rear of sticky labels,which may be printed on the front with data, such as for example a barcode, or other product identifiers. Apparatus for applying RFID tags tothe rear of labels is described in U.S. Pat. No. 6,280,544 B1, where thefront of the labels are printed and then the tags are applied to therear. The apparatus further includes a read station down stream of thetag being applied to the rear of the label, where the operation of thetag is checked before the label is dispensed from the apparatus for use.

In such prior applications, the tag is easy to locate with a read/writedevice since the tag itself is physically large enough to be found,and/or is in a constant position, for example when applied to a label,and in many cases the range of the read/write head is sufficientlylarge, for example tens of centimeters at least. Where the tag isminiaturized and of comparatively low power however, a problem thenarises in locating a read/write device sufficiently close to the tag forreading and/or writing of data to or from the tag to occur. An aim ofthe invention is to reduce or overcome the above problem.

SUMMARY OF THE INVENTION

According to one aspect of the invention, we provide a detector fordetecting the presence of a memory tag, the detector comprising a radiofrequency source operable to generate a radio frequency signal and adetector resonant circuit part connected to the radio frequency source,the detector resonant circuit part comprising an antenna, the detectorfurther comprising a power monitor responsive to the power of areflected signal returned from the detector resonant circuit part, thepower monitor being operable to generate an output in response to thepower of the reflected signal, wherein a decrease in the power of thereflected signal indicates the presence of a tag in the vicinity of theantenna.

The power monitor may be operable to generate an output when the powerlevel falls below a threshold.

The power monitor may be operable to generate an output indicating thepower of the reflected signal.

The position processor may be operable to receive position informationindicating the position of the detector and an output from the powermonitor, the position monitor being operable to store positioninformation relating to at least one position and the power monitoroutput at that position, and generate a recommended position outputdepending on the store position information and power monitor outputinformation.

According to a second aspect of the invention, we provide a read/writedevice for reading and/or writing data to a tag, the read/write devicecomprising a detector according to any one of claims 1 to 4, theread/write device being operable to read data from and/or write data tothe tag in response to the power monitor output.

The read/write system may comprise a detector and a movable head may beprovided with the antenna, the read/write system may be operable to movethe movable head and generate position information corresponding to theposition of the movable head, the read/write system further may beoperable to transmit the position information to the detector, receive arecommended position output from the detector and move the movable headto a position indicated by the recommended position output.

The read/write system may be operable to read data from and/or writedata to the tag when the movable head is at the position indicated bythe recommended position information.

The read/write system may comprise a printer, wherein the movable headmay comprise a print head and wherein the printer is operable to printon a base medium provided with at least one tag.

According to a third aspect of the invention we provide a printeroperable to print on a base medium provided with at least one tag, theprinter comprising a detector for detecting the presence of a memorytag, the detector comprising a radio frequency source operable togenerate a radio frequency signal and a detector resonant circuit partconnected to the radio frequency source, the detector resonant circuitpart comprising an antenna, the detector further comprising a powermonitor responsive to the power of a reflected signal returned from thedetector resonant circuit part, the power monitor being operable togenerate an output in response to the power of the reflected signal,wherein a decrease in the power of the reflected signal indicates thepresence of a tag in the vicinity of the antenna.

The printer may be operable to read data from and/or write data to thetag in response to the power monitor output.

The printer may have a movable print head, where the print head may beprovided with the antenna, the printer being operable to move themovable head and generate position information corresponding to theposition of the movable head, the printer further being operable totransmit the position information to the detector, receive a recommendedposition output from the detector and move the movable head to aposition indicated by the recommended position output.

According to a fourth aspect of the invention, we provide a method fordetecting the presence of a tag, comprising transmitting a signal to aresonant circuit part comprising an antenna, monitoring the power of areflected signal reflected from the resonant circuit part, wherein adecrease in the power of the reflected signal indicates the presence ofa tag in the vicinity of the antenna.

The method may comprise the step of moving a detector provided with theantenna relative to the tag, storing position information relating tothe position of the detector and power information related to the powerreflected signal at that position, and generating a recommended positionin accordance with store information.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the present invention will now be described by way ofexample only with reference to the accompanying drawings wherein;

FIG. 1 is a circuit diagram of a known read/write device and tag,

FIG. 2 a is a diagrammatic illustration of a read/write device embodyingthe present invention in the absence of a tag,

FIG. 2 b is a diagrammatic illustration of a read/write device embodyingthe present invention in the presence of a tag,

FIG. 3 is a diagrammatic illustration of a particular implementation ofa read/write device and tag embodying the present invention,

FIG. 4 is a diagrammatic illustration of a read/write system comprisinga printer embodying the present invention,

FIG. 5 is a flow diagram illustrating a method embodying the presentinvention, and

FIG. 6 is a diagrammatic illustration of a further read/write systemembodying the present invention.

FIG. 7 is a diagrammatic illustration of a further read/write systemembodying the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIGS. 2 a and 2 b, a detector embodying the presentinvention is shown generally at 23. The detector 23 in like manner tothe reader of FIG. 1 comprises a resonant circuit part 11′ comprising acapacitor 13′ and a inductor 14′. In this example, the inductor 14′comprises an antenna. Connected to the resonant circuit part 11′ is asignal generator 13′ operable to supply a signal at a desired frequencyto the resonant circuit part 11′.

To monitor the power reflected by the resonant circuit part 11, acoupler 24 is provided connected to a power monitor 25. When no tag iswithin the vicinity of the inductive part 14′, the signal generated bythe frequency source 13′ will be reflected by the resonant circuit part11. A part of this reflected signal will be split off by the coupler 15and directed to the power monitor 25.

A tag is shown at 12 identical to that of FIG. 1. The tag 12 simplycomprises a tag resonant circuit part 16 and a rectifying circuit part17 to supply power to a memory 18. The resonant circuit parts 11′, 16are tuned to have generally the same resonant frequency. When the tag 12is sufficiently close to the inductor 14′ such that there is inductivecoupling between the resonant circuit parts 11′, 16, the resonantcircuit part 16 will draw power from the magnetic field of the inductor,and the resulting signal is rectified by the rectifying circuit part 17to power the memory 18.

When the tag 12 is brought in the vicinity of the inductor 14′ as shownin FIG. 2 b the inductive coupling between the resonant circuit part 11′and the tag resonant circuit part 16 will draw power from the detector23, causing a reduction in the power of the signal reflected by theresonant circuit part 11′. This fall will be detected by the powermonitor 25 as shown diagrammatically in FIG. 2 b. Thus, by detecting thefall in the power of the reflected signal, the presence of a tag may beinferred.

A detector as shown in FIGS. 2 a and 2 b may advantageously beincorporated in a read/write device operable to retrieve data fromand/or write data to a tag.

Referring now to FIG. 3, a memory tag embodying the present invention isshown at 30 and a read/write device shown at 31. The tag 30 comprises aresonant circuit part 32 and a rectifying circuit part 33, together witha memory 18. The resonant circuit part 32 comprises an inductor L2 35and a capacitor C2 36 connected in parallel in like manner to the tag 12of FIG. 1, FIG. 2 b. The resonant circuit part 32 further comprises acontrollable capacitive element generally indicated at 37, in theexample of FIG. 3 comprising a capacitor C3 38 and a switch S1 39 whichis connected to a read data line 39 a connected to the memory 36 tomodulate the resonant frequency of the resonant circuit part 32. Therectifying part 33 comprises a diode D1 40 connected to the resonantcircuit part 32 in a forward biased direction and a capacitor C4 41connected in parallel with the components of the resonant circuit part32. The rectifying circuit part 33 operates in like manner to therectifying circuit part 17 of FIG. 1 as a half-wave rectifier to providepower to the memory 18. The tag 30 further comprises a write datacircuit part 42. The write data circuit part 42 comprises a diode D2 43connected in the forward bias direction to the output of the resonantcircuit part 32, with a capacitor C5 44 and a resistor R1 46 connectedin parallel with the components of the resonant circuit part 32. Thewrite data circuit part 42 thus in this embodiment comprises a simpleenvelope detector which is responsive to the magnitude of the signalgenerated by the resonant circuit part 32, and provides a write datasignal on a line 47 to the memory 18.

The read/write device 31 in like manner to the detector 23 comprises aresonant circuit part 50 which comprises an inductor L1 51 and acapacitor C1 54 connected in parallel. A frequency generator 53 isconnected to the resonant circuit part 50. The read/write device 31further comprises an amplitude modulator 54 which is controllable inresponse to data sent on a write data line 55. The amplitude modulator54 controls the power of the signal from the frequency generator 53 tothe resonant circuit part 50, and thus provides modulation of theamplitude of the power of the signal generated by the resonant circuitpart 32 which can be detected by the right data circuit part 42 of thetag 30.

The read/write device 31 further comprises a demodulator, generallyshown at 56. The demodulator 46 comprises a splitter 57 connected to thefrequency generator 45 to split off a part of the signal to provide areference signal. A coupler 58 is provided to split off a part of thereflected signal reflected back from the resonant circuit part 50. Thereference signal and reflected signal are passed to a multiplierindicated at 59. The multiplier 59 multiplies the reflected signal andthe reference signal and passes the output to a low pass filter 60. Thelow pass filter 60 passes a signal corresponding to the phase differencebetween the reference signal and the reflected signal to an output 61.By controlling the switch S1 39 of the tag 30 under control of thememory 34, the resonant frequency of resonant circuit part 32 can bemodulated and hence the phase of the reflected signal reflected by theresonant circuit part 50 with respect to a reference signal can bemodulated. This change of phase is detected by the demodulator 55, andso data can be read from the tag by the read/write device 31. By thismethod, data may be transmitted from the tag 30 whilst not significantlyaffecting the power drawn by the resonant circuit part 32.

A power monitor 62 is provided connected to the coupler 57 in likemanner to the coupler 24 of FIG. 2 a. Because, as describedhereinbefore, the tag 30 is operable to provide a modulated signal tothe read/write device without significantly varying the power drawn bythe resonant circuit part 32, the amplitude of the reflected signal fromthe resonant circuit part 50 will be primarily dependent on the powerdrawn by the tag 30 and be independent of the modulation of thereflected signal by operation of switch S1. Thus, the power monitor 62is operable to detect the presence of a tag 30 adjacent to theread/write device 31 by detecting a drop in the power in the signalreflected by the resonant circuit part 50. The power monitor may beimplemented in a relatively simple fashion, for example by providing asimple rectifier similar to the rectifying circuit part 33 of the tag30, to provide an output signal 62 comprising, in this example, a simplevoltage. An output is provided to a position processor shown at 63,which in FIG. 3 is shown as part of the read/write device 31 which it isapparent may be provided on any other device or element as appropriate.

The read/write device 31 of FIG. 3 may be used in an appropriateapplication as desired. For example, as shown with reference to FIG. 4,a printer is shown schematically at 70 provided with means to handle andprint on a base medium for example a roller shown diagrammatically at 71and a print head 72 movable on a track 73. The printer 70 is operable toreceive a suitable flexible base medium, in this example a sheet ofpaper 74, provided with a plurality of tags 30 distributed over thepaper 74. In this example, the print head 72 is provided with aread/write device 31, and the inductor 51 comprises a loop antennaprojecting from the print head 72. The printer comprises a printercontroller 75 provided with an external connection 76, for example to acomputer to receive data to be printed. The printer controller 75 isoperable to control the rollers 71 to feed the paper sheet 74, and alsoto move the print head 72 along the track 73 and instruct the print head72 to print on the paper 34.

In this embodiment, the printer controller 75 will be aware of theposition of the paper sheet 74 by virtue of operation of the rollerassembly 71 and also knows the position of the print head 72 on thetrack 73. This information is provided to the position processor 64,which will therefore know the position of the antenna 51 relative to thesheet of paper 74, and will also receive the output signal 63 from thepower monitor 62 at that position. As seen in FIG. 5, the positionprocessor will then be able to perform the method of, at step 80,receiving the position information, in this example the print headposition 32 and also the position of the paper. At step 81, theprocessor 63 will receive the power level information, that is the powersignal 63 received from the position monitor 62. As shown at step 82,the position processor can repeat the process as necessary, storinginformation as shown at step 83. The print head 72 or paper 74 may thenbe moved at step 84, for example by the position processor sending anoutput on line 65 to the printing controller 75 or otherwise as desired.When the position processor 64 has sufficient information, it maygenerate a recommended position output indicating an appropriateposition of a tag 30.

The method of FIG. 5 may be implemented as desired, further to provide agood as coupling as possible between the read/write device 31 and a tag30, or to provide a simple criteria as desired or otherwise. Forexample, it will be apparent that by performing the steps of the methodof FIG. 5 repeatedly, moving the print head and paper 84 as necessary, atwo-dimensional “map” of the sheet of paper 74 may be built of where theposition of tags 30 is indicated by minima in the power of the signalreflected from the resonant circuit part 50. Since the position of eachof the minima corresponding to a tag will be known, by sending theappropriate instructions to the printer controller 75 the roller 71 andthe print head 72 may be moved to position the antenna 51 directly overa tag 30 as desired. In a more simple implementation, the positionprocessor 63 will simply detect when the power of the reflected signalfalls below a given level, indicating that the tag 30 is sufficientlyclose to the antenna 51 to provide a sufficient inductive coupling,without necessarily taking further steps to identify the position of thetag 30. This might be applicable for circumstances where the print head72 is repeatedly scanned across the paper 74 is successive lines, anddata is written to be read from the tag 30 on each successive passduring the period when the reflected power signal is below a threshold.In this case, the step of storing the position and power information maybe omitted, or the position and power information may be stored to allowthe tag 30 to be relocated if desired.

As shown in FIG. 6, an alternative read/write device 70′ comprising aprinter is shown comprising an arm 80 provided with a plurality ofdetectors 81 similar to the detector 23 each operable to scan a part ofthe base medium, in this example paper 74′ as passes under the pluralityof detectors 81 and provide an output accordingly to the printercontroller 75′. This embodiment is particularly suitable for use in aread/write device such as a printer which does not have a scanning printhead.

In a yet further read/write system embodying the invention, a read/writesystem comprising a wand 90 is shown in FIG. 7, which may be manuallypositioned to attach a tag 30 on a sheet of paper 74, for example by auser touching the wand 90 to a printed mark. The wand 90 may comprise asimple detector as shown in FIGS. 2 a and 2 b, with a connection 92, forexample for connection to a computer, or may be provided with some orall of the functionality of the read/write device 31 and its owninternal memory to be able to operate without any such connection 92.

The wand 90 is provided with an antenna such as the antenna 51 of FIG.3, at its tip 91.

In the present specification “comprise” means “includes or consists of”and “comprising” means “including or consisting of”.

The features disclosed in the foregoing description, or the followingclaims, or the accompanying drawings, expressed in their specific formsor in terms of a means for performing the disclosed function, or amethod or process for attaining the disclosed result, as appropriate,may, separately, or in any combination of such features, be utilized forrealizing the invention in diverse forms thereof.

1. A printer operable to print on a base medium provided with at leastone memory tag, the printer comprising a detector for detecting thepresence of a memory tag, the detector comprising a radio frequencysource operable to generate a radio frequency signal and a detectorresonant circuit part connected to the radio frequency source, thedetector resonant circuit part comprising an antenna, the detectorfurther comprising a power monitor responsive to the power of areflected signal returned from the detector resonant circuit part, thepower monitor being operable to generate an output in response to thepower of the reflected signal, wherein a decrease in the power of thereflected signal indicates the presence of a tag in the vicinity of theantenna.
 2. A printer according to claim 1 wherein the printer isoperable to read data from and/or write data to the tag in response tothe power monitor output.
 3. A method for detecting the presence of atag, comprising transmitting a signal to a resonant circuit partcomprising an antenna, monitoring the power of a reflected signalreflected from the resonant circuit part, wherein a decrease in thepower of the reflected signal indicates the presence of a tag in thevicinity of the antenna.